Belgian Pat. No. 828,074 discloses that catalysts containing phosphorus, molybdenum, bismuth, copper, oxygen, at least one element selected from the group consisting of Fe, Co, Ni, and at least one element selected from the group consisting of Li, Na, Bi, Cs, Be, Mg, Sr and Ba are useful in the preparation of maleic anhydride from butene-1, butene-2, butadiene, pentane, pentadiene, cyclopentadiene and benzene.